A Krav Maga gun disarm involves twisting the body while redirecting the shot. Grabbing the gun isn't supposed to prevent the gun from firing, it's supposed to prevent the shot from killing you. In a real life situation when someone's got a gun to your head you probably don't have time to think about the gun's mechanics.
Well thats why you're thinking about it ahead of time.
I distinctly recall the instructors commenting they'd rather face an idiot who gets too close to you with a gun than an idiot who gets too close to you with a knife, the knife will still cut you if you grab it.
They also suggested you comply with anyone who has a weapon unless you have good reason to think they'll kill you anyway.
I Really should take Krav up again, was good stuff.
Yeah, I'm only a yellow belt at my school (Krav Maga Canada has a belt system even though the international org. doesn't) so we haven't gotten into weapon disarms much yet, but I think the general idea is it's easier to give up your wallet than successfully perform a gun disarm or redirect a knife.